Filter medium and filter unit

ABSTRACT

A filter medium includes a first porous PTFE membrane, a first air-permeable support member, a second porous PTFE membrane, and a second air-permeable support member. A surface of the filter medium is formed by the first porous PTFE membrane. A bond strength, as measured by a 180° peel test, between the first porous PTFE membrane and the first air-permeable support member is higher than 1.2 N/25 mm. The first air-permeable support member is an embossed non-woven fabric having one or more recessed portions and one or more projecting portions. A ratio of the area of the recessed portion or the total area of the recessed portions to the area of the embossed non-woven fabric serving as the first air-permeable support member is more than 15%.

TECHNICAL FIELD

The present invention relates to filter media and filter units.

BACKGROUND ART

Filter media having a porous polytetrafluoroethylene (PTFE) membrane are used in various applications, such as in intake air filters for turbines, air filters for clean rooms, and filters for household electric appliances. Patent Literature 1 discloses a filter medium including one support member and two porous PTFE membranes, the support member being held between the porous PTFE membranes. Patent Literature 2 discloses a filter medium including two support members and one porous PTFE membrane, the porous PTFE membrane being held between the support members. Patent Literature 1 also discloses a filter medium including two porous PTFE membranes and two support members, the porous PTFE membranes and the support members being alternately arranged.

In some cases, a filter medium is required to permit removal of dust from a surface of the filter medium so that the filter medium can be repeatedly used. As stated in Patent Literature 1, dust adhering to a surface of a filter medium can easily be removed when the surface of the filter medium is formed by a porous PTFE membrane.

CITATION LIST

Patent Literature 1: JP 2005-246233 A

Patent Literature 2: JP 2012-228687 A

SUMMARY OF INVENTION Technical Problem

To remove dust adhering to a surface of a filter medium, the filter medium may be exposed to a blast of air or may be washed with water. In this case, however, a porous PTFE membrane used in the filter medium may be damaged by a high pressure imposed by an air stream or water stream, since such a porous PTFE membrane is very thin. Specifically, the porous PTFE membrane may be broken or may be separated from a support member. Enhancing the bond strength between the porous PTFE membrane and the support member or increasing the thickness of the porous PTFE membrane improves the durability of the filter medium, but can lead to a significant decrease in air permeability (increase in pressure drop). That is, there is a trade-off relationship between the durability and air permeability.

In view of the above circumstances, the present invention aims to provide a technique by which the durability of a filter medium designed to permit easy removal of dust by cleaning can be improved while avoiding a significant decrease in air permeability. The present invention also aims to provide a filter unit including the filter medium.

Solution to Problem

That is, the present invention provides a filter medium comprising a first porous PTFE membrane, a first air-permeable support member, a second porous PTFE membrane, and a second air-permeable support member that are stacked in this order and bonded to one another, wherein

a surface of the filter medium is formed by the first porous PTFE membrane,

a bond strength, as measured by a 180° peel test, between the first porous PTFE membrane and the first air-permeable support member is higher than 1.2 N/25 mm,

the first air-permeable support member is an embossed non-woven fabric having one or more recessed portions and one or more projecting portions, and

a ratio of the area of the recessed portion or the total area of the recessed portions to the area of the embossed non-woven fabric serving as the first air-permeable support member is more than 15%.

In another aspect, the present invention provides a filter unit including:

the filter medium as defined above; and

a frame supporting an outer peripheral portion of the filter medium.

Advantageous Effects of Invention

According to the present invention, the durability of a filter medium designed to permit easy removal of dust by cleaning can be improved while avoiding a significant decrease in air permeability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a filter medium according to an embodiment of the present invention.

FIG. 2A is a plan view of an embossed non-woven fabric (T-type) suitable for use in the filter medium shown in FIG. 1.

FIG. 2B is a plan view of another embossed non-woven fabric (S-type) suitable for use in the filter medium shown in FIG. 1.

FIG. 2C is a diagram for describing an advantage of a T-type embossed non-woven fabric.

FIG. 3A is a schematic diagram illustrating a method of a 180° peel test for measuring a bond strength between a first porous PTFE membrane and a first air-permeable support member.

FIG. 3B is a schematic diagram illustrating a method of a 180° peel test for measuring a bond strength between a first air-permeable support member and a second porous PTFE membrane.

FIG. 4A is a diagram illustrating a thermal lamination process in the production of the filter medium shown in FIG. 1.

FIG. 4B is a diagram illustrating another exemplary thermal lamination process.

FIG. 5 is a perspective view of a filter unit including the filter medium shown in FIG. 1.

FIG. 6 is a configuration diagram of a cyclone vacuum cleaner including filter units.

FIG. 7 is a schematic diagram illustrating a method for testing the durability of filter media.

FIG. 8A is an optical photograph of a surface of a filter medium of Example 1 as observed after a test for examining the ease of cleaning.

FIG. 8B is an optical photograph of a surface of a filter medium of Comparative Example 4 as observed after a test for examining the ease of cleaning.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The present invention is not limited to the embodiment described below.

As shown in FIG. 1, a filter medium 20 according to the present embodiment includes a first porous PTFE membrane 11, a first air-permeable support member 12, a second porous PTFE membrane 13, and a second air-permeable support member 14. The first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are stacked in this order and are bonded to one another.

In the present embodiment, one surface of the filter medium 20 is formed by the first porous PTFE membrane 11. Porous PTFE membranes generally have a smooth, slippery surface. The configuration of the present embodiment thus makes it possible, when dust is deposited on the surface of the filter medium 20, to easily remove the dust from the surface of the filter medium 20.

In the present embodiment, the filter medium 20 is composed of four layers. The filter medium 20 may alternatively be composed of more than four layers. A filter medium composed of more than four layers can be obtained by alternately stacking porous PTFE membranes and air-permeable support members. To allow easy removal of dust, it is important that at least one surface (main surface) of the filter medium 20 be formed by a porous PTFE membrane. Specifically, the filter medium 20 has a main surface that faces upstream in the flow direction of gas to be filtered and a main surface that faces downstream in the flow direction, and the upstream main surface is formed by the first porous PTFE membrane 11. In the present embodiment, the downstream main surface is formed by the second air-permeable support member 14. The term “main surface” refers to that surface of the filter medium 20 which has the largest area.

The parameters such as thickness, surface density, average pore diameter, and porosity of the first porous PTFE membrane 11 may be equal to those of the second porous PTFE membrane 13. This means that porous PTFE membranes of the same type (porous PTFE membranes produced using the same material under the same conditions) can be used as the first porous PTFE membrane 11 and the second porous PTFE membrane 13. It should be understood that the parameters such as thickness, surface density, average pore diameter, and porosity of the first porous PTFE membrane 11 may be different from those of the second porous PTFE membrane 13.

A porous PTFE membrane that can be used as the first porous PTFE membrane 11 or as the second porous PTFE membrane 13 has an average pore diameter in the range of, for example, 0.01 to 100 μm or 0.01 to 50 μm. The porous PTFE membrane has a thickness, for example, in the range of 1 to 300 μm or 2 to 100 μm.

The porous PTFE membrane can be produced by the following method. First, a fine PTFE powder is mixed with a solvent to prepare a paste. The paste is extruded into the form of a sheet. The resulting PTFE sheet is stretched and sintered, and thus the porous PTFE membrane is obtained. In the stretching (typically biaxial stretching) of the PTFE sheet, the area stretch ratio (a value calculated by multiplying the stretch ratio in one axial direction by the stretch ratio in a direction perpendicular to the one axial direction) is, for example, in the range of 50 to 900. The term “PTFE” as used herein is intended to include “modified PTFE”.

The first air-permeable support member 12 and the second air-permeable support member 14 have a sheet shape. The first air-permeable support member 12 and the second air-permeable support member 14 each have higher strength and higher air permeability than, for example, a porous PTFE membrane used as at least one of the first porous PTFE membrane 11 and second porous PTFE membrane 13. As the first air-permeable support member 12 and the second air-permeable support member 14 there can be used woven fabrics, non-woven fabrics, meshes, nets, and foamed materials. Among these, non-woven fabrics are most preferably used. The fibers constituting the non-woven fabric used may be synthetic fibers made of a polymer material such as polyolefin (such as polyethylene and polypropylene), polyester (such as polyethylene terephthalate), polyamide, acrylic, and polyimide. The non-woven fabric may be a composite fabric constituted by a plurality of types of fibers. Non-woven fabrics of the same type (the same product number) or of different types may be used as the first air-permeable support member 12 and the second air-permeable support member 14. The first air-permeable support member 12 and the second air-permeable support member 14 each have a thickness, for example, in the range of 50 to 300 μm.

In the present embodiment, the first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are bonded to one another. The method for bonding these components is not particularly limited. These components may be bonded using an adhesive or may be bonded by thermal lamination. When the air-permeable support members 12 and 14 are non-woven fabrics having thermal adhesiveness, thermal lamination is suitable for the present embodiment. With the use of thermal lamination, it is easier to achieve sufficient bond strength between the porous PTFE membranes and the air-permeable support members while preventing a decrease in air permeability.

For example, when a non-woven fabric contains fibers made of a thermoplastic resin such as polyethylene, the non-woven fabric exhibits thermal adhesiveness at relatively low temperatures. When such a non-woven fabric and a porous PTFE membrane are placed on each other and they are subjected to a pressure under heating, part of the fibers of the non-woven fabric are melted and solidified, so that the non-woven fabric is bonded to the porous PTFE membrane. The bonding points are located only on the fibers of the non-woven fabric, and thus regions devoid of fibers can have air permeability.

The non-woven fabric may be an embossed non-woven fabric. The embossed non-woven fabric is a non-woven fabric having one or more recessed portions and one or more projecting portions. The embossed non-woven fabric has higher stiffness and higher strength than an unembossed non-woven fabric as thick as the embossed non-woven fabric. The embossed non-woven fabric has an indented pattern or, in other words, the embossed non-woven fabric has a sea-island structure when viewed in plan. In the embossed non-woven fabric of the type shown in FIG. 2A (T-type), elliptical portions corresponding to “islands” (where the fibers are not melted) are projecting portions, and a portion corresponding to the “sea” (where the fibers are melted) is a recessed portion. The T-type embossed non-woven fabric typically has a single, continuous recessed portion and a plurality of projecting portions. The T-type embossed non-woven fabric may have a plurality of recessed portions separate from each other. In the embossed non-woven fabric of the type shown in FIG. 2B (S-type), circular portions corresponding to “islands” (where the fibers are melted) are recessed portions, and a portion corresponding to the “sea” (where the fibers are not melted) is a projecting portion. The S-type embossed non-woven fabric typically has a plurality of recessed portions and a single, continuous projecting portion. The S-type embossed non-woven fabric may have a plurality of projecting portions separate from each other. With the use of these embossed non-woven fabrics, the locations of the bonding points described above are further limited, so that it is easier to achieve both high air permeability and high bond strength.

As described with reference to FIG. 2A and FIG. 2B, an embossed portion of an embossed non-woven fabric is formed as a result of melting and solidification of a part of fibers of the non-woven fabric. The embossed portion also has air permeability; however, the air permeability of the embossed portion is lower than the air permeability of an unembossed portion. It is inferred that the air permeability of the filter medium decreases with an increase in the area of the embossed portion. In other words, it is inferred that the air permeability of the filter medium decreases with an increase in the ratio of the area of the embossed portion to the area of the embossed non-woven fabric (embossed area ratio). This inference is sufficiently valid when the bond strength between the porous PTFE membrane and the embossed non-woven fabric is relatively low. However, this inference is not necessarily valid when the bond strength between the porous PTFE membrane and the non-woven fabric is relatively high.

Enhancement of the bond strength between the porous PTFE membrane and the non-woven fabric requires, for example, increasing the heat applied to the porous PTFE membrane and the non-woven fabric (raising the heating temperature and/or lengthening the heating time) in the thermal lamination process. In other words, it is necessary to melt the fibers of the non-woven fabric to a greater extent. As demonstrated by the results presented in EXAMPLES below, when the embossed area ratio in the non-woven fabric is small, increasing the heat applied to the porous PTFE membrane and the non-woven fabric causes a considerable decrease in the air permeability of the filter medium. By contrast, when the filter medium is fabricated under the same conditions except for the use of a non-woven fabric having a large embossed area ratio, the decrease in air permeability of the filter medium can be prevented. That is, it is important to use a non-woven fabric having an embossed area ratio appropriate for the required bond strength between the porous PTFE membrane and the non-woven fabric.

In the present embodiment, the filter medium 20 can be placed at a given location (for example, within a vacuum cleaner) in such a manner that dust will be deposited on the surface formed by the first porous PTFE membrane 11. To remove the dust adhering to the surface of the filter medium 20, the filter medium 20 needs to be washed with water, exposed to a blast of air, or rubbed with a brush. In the filter medium 20 of the present embodiment, a bond strength A₁, as measured by a 180° peel test, between the first porous PTFE membrane 11 and the first air-permeable support member 12 is higher than 1.2 N/25 mm. When the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 is higher than 1.2 N/25 mm, the filter medium 20 of the present embodiment exhibits sufficient resistance to pressure imposed by a water stream, air stream, or brush.

Furthermore, in the filter medium 20 of the present embodiment, the ratio of the area of a recessed portion (or the ratio of the total area of a plurality of recessed portions) of an embossed non-woven fabric serving as the first air-permeable support member 12 to the area of the embossed non-woven fabric is more than 15%. Thus, in the present embodiment, not only is the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 sufficiently high, but also the embossed area ratio in the non-woven fabric serving as the first air-permeable support member 12 is appropriately adjusted so that the increase in the pressure loss across the filter medium 20 due to the increase in the bond strength A₁ can be effectively prevented.

The term “embossed area ratio” as used herein to describe an embossed non-woven fabric refers to the ratio of the area of a recessed portion (portion where fibers are melted) or the total area of a plurality of recessed portions to the area of the embossed non-woven fabric. The embossed area ratio can be calculated by the following method. The surface of the non-woven fabric is observed with an electron microscope (SEM) at a given magnification (for example, a magnification of 25 times). On the basis of the obtained SEM image, the proportion of the embossed portion(s) (recessed portion(s)) is calculated. In the case of an S-type non-woven fabric (see FIG. 2B), the recessed portions are assumed to be circular. In the case of a T-type non-woven fabric (see FIG. 2A), the projecting portions (unembossed portions) are assumed to be elliptical.

The embossed area ratio in the non-woven fabric serving as the first air-permeable support member 12 can be measured, even after production of the filter medium 20, by separating the first porous PTFE membrane 11 or second porous PTFE membrane 13 from the first air-permeable support member 12. With the use of a flat roll having a flat surface in the thermal lamination process, the embossed area ratio in the non-woven fabric hardly changes even after the thermal lamination process. The embossed portion(s) can be distinguished from the rest of the non-woven fabric even after separation of the first porous PTFE membrane 11 or second porous PTFE membrane 13 from the first air-permeable support member 12. The same applies to the second air-permeable support member 14.

The embossed non-woven fabric is, for example, a double-embossed non-woven fabric having two embossed surfaces. When a double-embossed non-woven fabric is used as the first air-permeable support member 12, the two embossed surfaces of the first air-permeable support member 12 are in contact with the first porous PTFE membrane 11 and the second porous PTFE membrane 13, respectively. When double-embossed non-woven fabrics are used as the first air-permeable support member 12 and the second air-permeable support member 14, the use of the same material can be expected to produce a cost-reducing effect. Additionally, the use of double-embossed non-woven fabrics reduces the occurrence of errors in manufacturing of the filter medium 20, since there is no distinction between the two sides of double-embossed non-woven fabrics.

A single-embossed non-woven fabric having only one embossed surface may be used as the first air-permeable support member 12. A single-embossed non-woven fabric having only one embossed surface may be used as the second air-permeable support member 14.

The embossed area ratio is desirably more than 20%. The upper limit of the embossed area ratio is not particularly defined. The embossed area ratio may be less than 60% or may even be less than 50%. When the embossed area ratio in the first air-permeable support member 12 is within an appropriate range, the filter medium 20 having a good balance between the durability and air permeability can be obtained.

Embossed non-woven fabrics include T-type embossed non-woven fabrics and S-type embossed non-woven fabrics as described with reference to FIG. 2A and FIG. 2B. For the present embodiment, the use of a T-type embossed non-woven fabric (FIG. 2A) is recommended. The T-type embossed non-woven fabric has the advantage of being able to be easily pleated.

In the T-type embossed non-woven fabric shown in FIG. 2A, the longitudinal direction of a plurality of unembossed portions (the direction of the major axis of each ellipse) coincides with the vertical or horizontal direction. In other words, each pair of the unembossed portions form a T-shape. Thus, the T-type embossed non-woven fabric can easily be pleated when, as shown in FIG. 2C, the pleating is carried out in such a manner that the direction of the pleating folds (the direction of mountain and valley folds) coincides with the longitudinal direction of the unembossed portions 16.

In the present embodiment, the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 is higher than a bond strength A₂, as measured by the 180° peel test, between the first air-permeable support member 12 and the second porous PTFE membrane 13. The second porous PTFE membrane 13 is disposed between the pair of support members 12 and 14 and has no direct contact with a brush or water during cleaning of the filter medium 20. This is why the bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13 has no significant influence on the durability required for removal of dust from the surface of the filter medium 20. Thus, adjusting the bond strengths A₁ and A₂ to establish the above-mentioned relationship makes it possible to improve the durability of the filter medium 20 while avoiding a significant decrease in air permeability. That is, both high durability and high air permeability can be achieved.

In the present embodiment, the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 is desirably 1.8 N/25 mm or more. In this case, higher durability can be imparted to the filter medium 20. The bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 may be 2.5 N/25 mm or less. In this case, it is possible to avoid excessively close bonding and impart sufficient air permeability to the filter medium 20. When the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 is in the range of 1.8 N/25 mm to 2.5 N/25 mm, the filter medium 20 having a good balance between the durability and air permeability can be obtained.

The bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13 is not particularly limited, as long as the bond strength A₂ is lower than the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12. The upper limit of the bond strength A₂ is, for example, 1.6 N/25 mm. The lower limit of the bond strength A₂ is, for example, 0.2 N/25 mm. When the bond strength A₂ falls within an appropriate range, the filter medium 20 is more likely to have sufficient air permeability. The difference between the bond strength A₁ and the bond strength A₂ is not particularly limited either. In an example, the difference between the bond strength A₁ and the bond strength A₂ is in the range of 0.2 to 2.3 N/25 mm.

The above values of the bond strengths A₁ and A₂ are those measured by a 180° peel test. The 180° peel test can be conducted by the below-described methods according to Japanese Industrial Standard, JIS Z 0237.

The method illustrated in FIG. 3A is a method for measuring the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12. First, the filter medium 20 is cut into a test specimen with a size of 100 mm×25 mm. The test specimen has a length of 100 mm in the MD direction (MD: Machine Direction) of the first porous PTFE membrane 11 and a width of 25 mm in the TD direction (TD: Transverse Direction) of the first porous PTFE membrane 11. The MD and TD directions correspond to those in the production of the first porous PTFE membrane 11. A non-bonded portion where the first porous PTFE membrane 11 and the first air-permeable support member 12 are not bonded is provided beforehand at the longitudinal end of the test specimen so that peeling can occur at the interface between the first porous PTFE membrane 11 and the first air-permeable support member 12. Next, the test specimen is attached to a stainless steel plate 25 with a double-coated adhesive tape 26 (No. 500, manufactured by Nitto Denko Corporation). The non-bonded portion of the test specimen is then secured to a chuck 24 of a tensile tester (Autograph AG-1, manufactured by Shimadzu Corporation). After that, the chuck 24 is drawn upward at a speed of 300 mm/min to cause peeling at the interface between the first porous PTFE membrane 11 and the first air-permeable support member 12, thereby measuring the 180° peel strength. Values measured initially after the start of the measurement over a length of 25 mm are ignored, and the average of the subsequently measured values (in units of N) continuously recorded for a 50-mm-long portion of the test specimen stripped off from the stainless steel plate 25 is determined as the bond strength A₁ of the filter medium 20.

The method illustrated in FIG. 3B is a method for measuring the bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13. In this method, a non-bonded portion where the first air-permeable support member 12 and the second porous PTFE membrane 13 are not bonded is provided at the longitudinal end of the test specimen so that peeling can occur at the interface between the first air-permeable support member 12 and the second porous PTFE membrane 13. The non-bonded portion of the test specimen is secured to the chuck 24 of the tensile tester. After that, the chuck 24 is drawn upward at a speed of 300 mm/min to cause peeling at the interface between the first air-permeable support member 12 and the second porous PTFE membrane 13, thereby measuring the 180° peel strength. The average of measured values (in units of N) continuously recorded is determined as the bond strength A₂ of the filter medium 20. Also in the method illustrated in FIG. 3B, the test specimen has a length of 100 mm in the MD direction (MD: Machine Direction) of the second porous PTFE membrane 13 and a width of 25 mm in the TD direction (TD: Transverse Direction) of the first porous PTFE membrane 11. In general, the MD and TD directions of the second porous PTFE membrane 13 coincide with the MD and TD directions of the first porous PTFE membrane 11.

When measurement is performed by the method illustrated in FIG. 3A, cohesive failure may occur in the first porous PTFE membrane 11 without evident peeling at the interface between the first porous PTFE membrane 11 and the first air-permeable support member 12. However, the value obtained by the method described with reference to FIG. 3A is defined herein as the “bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12”. Similarly, when measurement is performed by the method illustrated in FIG. 3B, cohesive failure may occur in the second porous PTFE membrane 13 without evident peeling at the interface between the first air-permeable support member 12 and the second porous PTFE membrane 13. The value obtained by the method described with reference to FIG. 3B is defined herein as the “bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13”.

The pressure drop across the filter medium 20 is, for example, in the range of 50 to 400 Pa. The “pressure drop” refers to a pressure drop that occurs when air is allowed to pass through the filter medium 20 at a flow velocity of 5.3 cm/sec. Specifically, the pressure drop can be measured by the following method. That is, the filter medium 20 is set to a cylindrical holder with an effective area of 100 cm², and a pressure difference is created between the two sides of the filter medium 20 to allow air to pass through the filter medium 20. The flow velocity of the air passing through the filter medium 20 is adjusted to 5.3 cm/sec (corresponding to a flow rate of 31.8 m³/min) with the aid of a flowmeter, and then the pressure drop is measured with a pressure meter (manometer).

The filter medium 20 exhibits a collection efficiency higher than 90%, for example, for particles having a particle diameter in the range of 0.1 to 0.2 μm. Generally, a filter medium that exhibits high collection efficiency for small particles tends to have low air permeability, and it is difficult to impart both high durability and high air permeability to such a filter medium. According to the present embodiment, it is possible to impart both high durability and high air permeability to a filter medium that exhibits high collection efficiency for small particles.

The collection efficiency can be measured by the following method. That is, the filter medium 20 is set to a cylindrical holder with an effective area of 100 cm², and a pressure difference is created between the two sides of the filter medium 20 to allow gas to pass through the filter medium 20. The pressure difference is adjusted to control the liner velocity of the gas passing through the filter medium 20 to 5.3 cm/sec (corresponding to a flow rate of 31.8 m³/min). Next, polydisperse dioctyl phthalate (DOP) particles specified in JIS Z 8901 are introduced into the gas present upstream of the filter medium 20 in such a manner that the concentration of particles having particle diameters in a predetermined range is 10⁶ particles/liter. The concentration of the DOP particles present downstream of the filter medium 20 is then measured with a particle counter. The range of the diameter of the particles to be counted by the particle counter is, for example, from 0.1 to 0.2 μm. The collection efficiency can be calculated by the following equation: Collection efficiency=(1−(downstream concentration of DOP particles/upstream concentration of DOP particles))×100(%). Polyalphaolefin (PAO) may be used instead of the DOP particles.

The filter medium 20 may be a HEPA filter (High Efficiency Particulate Air Filter) or may be an ULPA filter (Ultra Low Penetration Air Filter). Both the HEPA filter and ULPA filter are filters specified in Japanese Industrial Standard, JIS Z 8122.

The filter medium 20 may be pleated into a series of W-shapes. The pleating of the filter medium 20 can be accomplished using a known pleating machine (such as a rotary pleating machine, a reciprocating pleating machine, and a striping pleating machine). In the filter medium 20 of the present embodiment, the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 is higher than the bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13. In this case, the stiffness of the filter medium 20 is lower than in the case where the bond strength A₂ is equal to the bond strength A₁, and thus the above pleating can more easily be carried out.

Next, a method for producing the filter medium 20 will be described.

As shown in FIG. 4A, the first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are individually prepared on rolls. The first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are fed from the rolls and assembled together to form a stack 20 s of the membranes and members. The stack 20 s is delivered to a pair of lamination rolls 27 a and 27 b and passed through the gap between the lamination rolls 27 a and 27 b. During this passage, heat and pressure are applied to the stack 20 s from the rolls 27 a and 27 b, and thereby the fibers constituting the air-permeable support members (non-woven fabrics) 12 and 14 are melted and solidified, so that the first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are bonded to one another. The filter medium 20 is thus obtained.

The pair of lamination rolls 27 a and 27 b is configured to apply heat and pressure to the stack 20 s. In the present embodiment, the rolls 27 a and 27 b are configured so that the surface temperature of the roll 27 a that contacts one surface of the stack 20 s can be made different from the surface temperature of the roll 27 b that contacts the other surface of the stack 20 s. For example, only the lamination roll 27 a is equipped with a heater, while the lamination roll 27 b is not equipped with any heater. Typically, the lamination roll 27 a is a heating roll, and the lamination roll 27 b is a nip roll. As the stack 20 s is passing through the gap between the lamination rolls 27 a and 27 b, the first porous PTFE membrane 11 contacts the lamination roll 27 a and the second air-permeable support member 14 contacts the lamination roll 27 b. This allows heat to be preferentially transmitted to the first porous PTFE membrane 11 and the first air-permeable support member 12. The thermal lamination of the stack 20 s can thus be accomplished so that the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 will be higher than the bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13. It is naturally important to appropriately control the conditions such as the surface temperature of the lamination roll 27 a, the pressure applied to the stack 20 s, and the conveyance speed of the stack 20 s.

Alternatively, the thermal lamination process can be carried out in two stages as shown in FIG. 4B. In the first stage, which is illustrated in the upper part of FIG. 4B, the stack 20 s is heated from both above and below by heaters 29 placed on the conveyance path, and is then directed to the gap between a roll 28 a and a roll 28 b. Typically, the roll 28 a is a rotating roll, and the roll 28 b is a nip roll. The heaters 29 are, for example, infrared heaters. For example, the rolls 28 a and 28 b have the function of applying pressure to the stack 20 s but do not have the function of applying heat to the stack 20 s. Thus, the first porous PTFE membrane 11, the first air-permeable support member 12, the second porous PTFE membrane 13, and the second air-permeable support member 14 are provisionally bonded through the rolls 28 a and 28 b, thereby giving a stack 20 k. Next, in the second stage, which is illustrated in the lower part of FIG. 4B, the stack 20 k is delivered to the lamination rolls 27 a and 27 b identical to those as described with reference to FIG. 4A. As the stack 20 k is passing through the gap between the lamination rolls 27 a and 27 b, the lamination roll 27 a for heating contacts the first porous PTFE membrane 11 of the stack 20 k. The thermal lamination of the stack 20 k can thus be accomplished so that the bond strength A₁ between the first porous PTFE membrane 11 and the first air-permeable support member 12 will be higher than the bond strength A₂ between the first air-permeable support member 12 and the second porous PTFE membrane 13.

In the methods described with reference to FIG. 4A and FIG. 4B, the first porous PTFE membrane 11 and the first air-permeable support member 12 may be preliminarily bonded loosely before the formation of the stack 20 s. Similarly, the second porous PTFE membrane 13 and the second air-permeable support member 14 may be preliminarily bonded loosely.

As shown in FIG. 5, a filter unit 30 includes a filter medium 20 a and a support frame 22. The filter medium 20 a is obtained by pleating of the filter medium 20 shown in FIG. 1. The support frame 22 supports the outer peripheral portion of the filter medium 20 a. The support frame 22 is made of resin or metal. The filter medium 20 a may be fixed to the support frame 22 with an adhesive. The support frame 22 may be provided with a structure for fixedly holding the outer peripheral portion of the filter medium 20 a. Furthermore, the outer peripheral portion of the filter medium 20 a may be buried in the support frame 22. That is, the support frame 22 and the filter medium 20 a may be integrally formed by insert molding.

Next, an example of the application of the filter unit 30 will be described. The filter unit 30 can be used, for example, as an exhaust filter of a vacuum cleaner.

In the example shown in FIG. 6, the vacuum cleaner 40 is a cyclone vacuum cleaner. However, the filter unit 30 of the present embodiment is applicable to a vacuum cleaner (such as a paper bag vacuum cleaner) other than cyclone vacuum cleaners.

The vacuum cleaner 40 includes: a mechanism 31 (a cyclone or paper bag) for separating dust from intake air; a motor 32 for rotating a fan 33; and at least one filter unit 30. In the example shown in FIG. 6, two (a plurality of) filter units 30 are provided. One of the filter units 30 is disposed between the mechanism 31 and the motor 32 in an air flow path. The other of the filter units 30 is disposed between the motor 32 and an exhaust port (not shown) in the air flow path. In each filter unit 30, the first porous PTFE membrane 11 is located most upstream in the flow direction of air and the second air-permeable support member 14 is located most downstream in the flow direction of air. Thus, dust is deposited mainly on the surface of the first porous PTFE membrane 11. One of the filter units 30 that is located more downstream serves also to collect carbon powder discharged from the motor 32. Each filter unit 30 is detachable from the vacuum cleaner 40. Dust deposited on the surface of each filter unit 30 (in particular, on the surface of the first porous PTFE membrane 11) can be removed, for example, by means of a brush, an air stream, or a water stream.

EXAMPLES Example 1

A filter medium having the configuration described with reference to FIG. 1 was fabricated by the method described with reference to FIG. 4A. ULPA-grade porous PTFE membranes (NTF 9522-01, manufactured by Nitto Denko Corporation) were used as the first porous PTFE membrane and the second porous PTFE membrane. T-type PET/PE core-sheath non-woven fabrics (T0303WDO, double-embossed non-woven fabric manufactured by UNITIKA LTD.; embossed area ratio=47%, melting point of sheath-forming PE=129° C., melting point of core-forming PET=261° C.) were used as the first air-permeable support member and the second air-permeable support member. The surface temperature of the lamination roll (roll 27 a) for heating was 200° C. A nip roll having no heater was used as the other lamination roll (roll 27 b). The conveyance speed of the stack (stack 20 s) of the first porous PTFE membrane, the first air-permeable support member, the second porous PTFE membrane, and the second air-permeable support member was 5 m/min.

Example 2

A filter medium having the configuration described with reference to FIG. 1 was fabricated by the method described with reference to FIG. 4B. ULPA-grade porous PTFE membranes (NTF 9522-01, manufactured by Nitto Denko Corporation) were used as the first porous PTFE membrane and the second porous PTFE membrane. T-type PET/PE core-sheath non-woven fabrics (T0303WDO, manufactured by UNITIKA LTD.) were used as the first air-permeable support member and the second air-permeable support member. In the first stage as illustrated in the upper part of FIG. 4B, a power supplied to the infrared heaters (heaters 29) was controlled so that the stack (stack 20 s) was heated at a temperature of 150° C. In the first stage, the conveyance speed of the stack was 7 m/min. In the second stage as illustrated in the lower part of FIG. 4B, the surface temperature of the lamination roll (roll 27 a) for heating was 150° C. A nip roll having no heater was used as the other lamination roll (roll 27 b). In the second stage, the conveyance speed of the stack (stack 20 k) of the first porous PTFE membrane, the first air-permeable support member, the second porous PTFE membrane, and the second air-permeable support member was 5 m/min.

Example 3

A filter medium of Example 3 was fabricated in the same manner as in Example 2, except for using an S-type PET/PE core-sheath non-woven fabric (S0303WDO, double-embossed non-woven fabric manufactured by UNITIKA LTD.; embossed area ratio=13%) as the second air-permeable support member.

Comparative Example 1

A filter medium of Comparative Example 1 was fabricated in the same manner as in Example 1, except for using S-type PET/PE core-sheath non-woven fabrics (S0303WDO, manufactured by UNITIKA LTD.) as the first air-permeable support member and the second air-permeable support member.

Comparative Example 2

A filter medium of Comparative Example 2 was fabricated in the same manner as in Example 2, except that, in the method described with reference to FIG. 4B, only the first stage (the upper part of FIG. 4B) was performed using S-type PET/PE core-sheath non-woven fabrics (S0303WDO, manufactured by UNITIKA LTD.) as the first air-permeable support member and the second air-permeable support member with omission of the second stage (the lower part of FIG. 4B).

Comparative Example 3

A filter medium of Comparative Example 3 was fabricated in the same manner as in Example 2, except that, in the method described with reference to FIG. 4B, only the first stage (the upper part of FIG. 4B) was performed with omission of the second stage (the lower part of FIG. 4B).

Comparative Example 4

A five-layer filter medium including a first air-permeable support member, a first porous PTFE membrane, a second air-permeable support member, a second porous PTFE membrane, and a third air-permeable support member that were stacked in this order and bonded to one another was fabricated in the same manner as in Comparative Example 2. That is, a five-layer filter medium of Comparative Example 4 was fabricated in the same manner as in Example 2, except that, in the method described with reference to FIG. 4B, only the first stage (the upper part of FIG. 4B) was performed with omission of the second stage (the lower part of FIG. 4B). ULPA-grade porous PTFE membranes (NTF 9522-01, manufactured by Nitto Denko Corporation) were used as the first porous PTFE membrane and the second porous PTFE membrane. T-type PET/PE core-sheath non-woven fabrics (T0303WDO, manufactured by UNITIKA LTD.) were used as the first to third air-permeable support members. The filter medium of Comparative Example 4 corresponds to a filter medium having a configuration disclosed in FIG. 1 of Patent Literature 2.

[Pressure Drop]

The pressure drop across each of the filter media of Examples and Comparative Examples was measured by the method previously described. The results are shown in Table 1.

[Collection Efficiency]

The collection efficiency of each of the filter media of Examples and Comparative Examples was measured by the method previously described. For this collection efficiency measurement, particles having a particle diameter in the range of 0.1 to 0.2 μm (particles whose particle diameters are distributed within the above range) were used. The results are shown in Table 1.

[Durability]

A durability test was conducted for the filter media of Examples and Comparative Examples by the following method. First, each of the filter media of Examples and Comparative Example was cut to give a test specimen having a length of 300 mm and a width of 900 mm. As shown in FIG. 7, air with a pressure of 0.2 MPa was blown to a surface of the test specimen (the surface formed by a porous PTFE membrane) by an air gun 35 at an angle of 45 degrees from a point 10 cm away from the surface. While air was blown to the test specimen, the air gun 35 was slowly moved in the width direction WD of the test specimen for 10 seconds. After that, the test specimen was visually inspected. When pealing of the porous PTFE membrane was not observed in the test specimen, the filter medium was rated as acceptable (∘), while when pealing of the porous PTFE membrane was observed in the test specimen, the filter medium was rated as unacceptable (x). The results are shown in Table 1. The width direction WD coincided with the TD direction in the production of the porous PTFE membrane.

[Ease of Cleaning]

A test for examining the ease of cleaning of the filter media of Examples and Comparative Examples was conducted by the following method. First, each of the filter media of Examples and Comparative Examples was cut to give a test specimen attachable to a circular-conical holder with an effective area of 100 cm². The test specimen was set to the holder, and 0.2 g of test powder (No. 8) specified in JIS Z 8901 was spread on the surface of the filter medium. Air was then allowed to pass through the test specimen at a linear velocity of 0.2 m/min for 1 minute. Subsequently, the surface of the test specimen was washed with running water for 5 minutes to remove the powder. This procedure was repeated five times. After that, the test specimen was visually inspected. When fouling was not clearly visible in the test specimen, the filter medium was rated as acceptable (∘), while when fouling was clearly visible in the test specimen, the filter medium was rated as unacceptable (x). The results are shown in Table 1, FIG. 8A, and FIG. 8B. FIG. 8A is an optical photograph of the surface of the filter medium of Example 1 as observed after the test. FIG. 8B is an optical photograph of the surface of the filter medium of Comparative Example 4 as observed after the test.

[Increase in Pressure Drop]

After the test for examining the ease of cleaning, each test specimen was thoroughly dried, and the pressure drop was then measured. The increase in pressure drop was calculated by the equation given below. The results are shown in Table 1.

Increase in pressure drop (%)=100×(P2−P1)/P1

P1: Pressure drop as measured before the test for examining the ease of cleaning

P2: Pressure drop as measured after the test for examining the ease of cleaning

[Bond Strength]

Each of the filter media of Examples and Comparative Examples was subjected to a 180° peel test. Specifically, the bond strength A₁ between the first porous PTFE membrane and the first air-permeable support member (non-woven fabric) was measured by the method described with reference to FIG. 3A. The bond strength A₂ between the first air-permeable support member (non-woven fabric) and the second porous PTFE membrane was measured by the method described with reference to FIG. 3B. For Comparative Example 4, a bond strength between the first air-permeable support member (first layer) and the first porous PTFE membrane (second layer) was measured as the “bond strength A₁” according to the method described with reference to FIG. 3B, and a bond strength between the second air-permeable support member (third layer) and the second porous PTFE membrane (fourth layer) was measured as the “bond strength A₂” according to the method described with reference to FIG. 3B. The results are shown in Table 1.

TABLE 1 Increase in Type of Thermal Pressure Collection pressure Bond Bond non-woven lamination drop efficiency Ease of drop strength A₁ strength A₂ fabric process (Pa) (%) Durability cleaning (%) (N/25 mm) (N/25 mm) Example 1 T/T One-stage 280 99.99≤ ∘ ∘ 0 2.5 1.0 process (heating roll) Example 2 T/T Two-stage 220 99.99≤ ∘ ∘ 0 2.1 0.4 process Example 3 T/S Two-stage 220 99.99≤ ∘ ∘ 0 2.1 0.4 process Comparative S/S One-stage 370 99.99≤ ∘ ∘ 0 2.6 1.5 Example 1 process (heating roll) Comparative S/S One-stage 230 99.99≤ x Not Not 1.2 0.4 Example 2 process measurable measurable (infrared) Comparative T/T One-stage 200 99.99≤ x Not Not 1.2 0.4 Example 3 process measurable measurable (infrared) Comparative T/T/T One-stage 220 99.99≤ Not tested x 6 0.4 0.4 Example 4 process (infrared)

As shown in Table 1, the filter medium of Example 1, in which a non-woven fabric having a large embossed area ratio was used as the first air-permeable support member, exhibited a sufficiently lower pressure drop than the filter medium of Comparative Example 1 in which a non-woven fabric having a small embossed area ratio was used as the first air-permeable support member.

As seen by comparing Example 1 and Example 2, the two-stage lamination process described with reference to FIG. 4B successfully increased the bond strength A₁ between the first porous PTFE membrane and the first air-permeable support member while reducing the increase in the bond strength A₂ between the first air-permeable support member and the second porous PTFE membrane.

As seen from Example 2, the filter medium of Example 2, in which a non-woven fabric having a large embossed area ratio was used as the first air-permeable support member, exhibited a sufficiently low pressure drop despite the use of the two-stage lamination process.

As seen by comparing Example 2 and Example 3, the difference in the embossed area ratio of the second air-permeable support member exerted no significant effect on the properties of the filter media.

For the filter media of Comparative Examples 2 and 3, the bond strength A₁ was as low as 1.2 N/25 mm, and peeling of the porous PTFE membrane was observed in the durability test.

The bond strength A₂ in Example 2 was approximately equal to the bond strength A₂ in Comparative Example 3. This suggests that the second stage (the lower part of FIG. 4B) in the method described with reference to FIG. 4B is capable of preferentially increasing the bond strength A₁ while causing little increase in bond strength A₂. That is, the method described with reference to FIG. 4B allows easy control of the bond strengths A₁ and A₂.

The surface (outermost surface) of each of the filter media of Examples 1 to 3 was formed by a porous PTFE membrane. Thus, as seen from FIG. 8A, dust was sufficiently removed by simple cleaning. By contrast, the surface of the filter medium of Comparative Example 4 was formed by an air-permeable support member (non-woven fabric). Thus, as seen from FIG. 8B, dust was not sufficiently removed by cleaning. The surface of each of the filter media of Comparative Examples 2 and 3 was formed by a porous PTFE membrane; however, the porous PTFE membrane was broken during cleaning because the bond strength A₁ was low. The bond strength A₁ of the filter medium of Comparative Example 4 was as low as 0.4 N/25 mm. However, the non-woven fabric forming the surface of the filter medium of Comparative Example 4 was not broken during cleaning.

The increase in pressure drop was 0% for all of the filter media of Examples 1 to 3. This means that, in Examples 1 to 3, the pressure drop remained unchanged before and after the test for examining the ease of cleaning. For the filter medium of Comparative Example 4, the pressure drop was increased by about 6%, compared to that measured before the test for examining the ease of cleaning.

INDUSTRIAL APPLICABILITY

The technique disclosed herein is applicable to various filters such as intake air filters for turbines, air filters for clean rooms, and filters for household electric appliances. The technique disclosed herein particularly contributes to improvement of vacuum-cleaner filters which may be frequently cleaned. 

1. A filter medium comprising a first porous polytetrafluoroethylene membrane, a first air-permeable support member, a second porous polytetrafluoroethylene membrane, and a second air-permeable support member that are stacked in this order and bonded to one another, wherein a surface of the filter medium is formed by the first porous polytetrafluoroethylene membrane, a bond strength, as measured by a 180° peel test, between the first porous polytetrafluoroethylene membrane and the first air-permeable support member is higher than 1.2 N/25 mm, the first air-permeable support member is an embossed non-woven fabric having one or more recessed portions and one or more projecting portions, and a ratio of the area of the recessed portion or the total area of the recessed portions to the area of the embossed non-woven fabric serving as the first air-permeable support member is more than 15%.
 2. The filter medium according to claim 1, wherein the embossed non-woven fabric has a sea-island structure when viewed in plan, and in the embossed non-woven fabric, a portion corresponding to the sea is the recessed portion, and a portion corresponding to an island is the projecting portion.
 3. The filter medium according to claim 1, wherein the ratio is less than 60%.
 4. The filter medium according to claim 1, wherein the bond strength between the first porous polytetrafluoroethylene membrane and the first air-permeable support member is higher than a bond strength, as measured by the 180° peel test, between the first air-permeable support member and the second porous polytetrafluoroethylene membrane.
 5. The filter medium according to claim 1, wherein the first porous polytetrafluoroethylene membrane, the first air-permeable support member, the second porous polytetrafluoroethylene membrane, and the second air-permeable support member are bonded to one another by thermal lamination.
 6. The filter medium according to claim 1, wherein the filter medium is pleated.
 7. A filter unit comprising: the filter medium according to claim 1; and a support frame supporting an outer peripheral portion of the filter medium. 